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The moment magnitude scale (MW) provides an estimate of the total energy released in an earthquake and is currently the preferred magnitude scale in use by seismologists .

It is calculated from the seismic moment (M0 - which has the unit of dynes/cm =where 1 dyne/cm = 1x10-7 N/m) which is a measure of the total energy released during an earthquake and is derived based on the elastic moduli of the crust where the earthquake occurred, the length of the slip surface and the cross sectional area of the slip surface as follows:

M0 = G x AFx DF

Where:

G = Shear modulus of the rock mass

AF = Area of the rupture along the fault

DF = average displacement on AF

In order to make the moment magnitude scale (Mw) consistent with older magnitude scales such as the Local Moment (or "Richter") scale the seismic moment (M0) is converted into a logarithmic scale using the following equation:

Mw = 2/3 x log10 x (M0) - 10.7

This ultimately yields a dimensionless number and as such, Moment Magnitude has no units.

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